Page:Quarterly Journal of the Geological Society of London, vol. 34.djvu/879

Rh PEESUMABLT THAT OP KHJANODON - MANTELLI. 747

remains of the cartilaginous rod upon which the bony elements of the jaw were laid down. All that part of the vertical plate which underlies and supports the tympanic surface is stout and sutural. The lower border of the vertical plate in front, where it is broken through, is 1 to 1^- inch wide ; and behind it tapers to nothing at the hinder end of the jaw. In its whole length this border is longi- tudinally grooved by sutural marks.

The horizontal plate bearing the tympanic surface, viewed from above, has a roughly triangular outline (fig. 2 ; fig. 3 a). Its inner border is almost straight : and for its anterior two thirds it projects inwards beyond the vertical plate in a shelf-like manner. It meets in front the anterior border, making with this an angle which, in a less incomplete example, ran forward for about an inch in a tongue-like form. The anterior border, rising and at the same time acquiring a slight outward twist, becomes a highly inclined curved sheet, which above, at its outer border, is separated by a shallow groove from the upper lip of the vertical plate.

The tympanic articular surface is hollow from side to side, and also from before backward. Its greatest length is in the direction of the axis of the jaw ; and in this direction, behind, a low median rising imperfectly marks off an inner and an outer part.

The above details will have made it apparent that the mandible represented by this articular bone differed greatly from that of Crocodilia, and in a less degree from those of extant lizards ; whilst in some important features it resembled that of Hypsilophodon Foxii. This, and the occurrence of the bone in relative abundance in the same beds and localities which have yielded Iguanodon-mandibles, always wanting the articular element, appear to me sufficient warrant for (at least provisionally) referring this os articulare to the Iguanodon.